usb: xhci: dbc: Fixing typo error.

Replaced "xhci_dbc_flush_reqests" with xhci_dbc_flush_requests".

Signed-off-by: Prabhat Chand Pandey <prabhat.chand.pandey@intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>